A liquid crystal display device commonly requires a separate light source such as a backlight as it is not a self-luminous device. A backlight unit chiefly includes a light source and a light guide plate guiding light emitted from the light source to realize surface emission.
In general, the backlight unit is categorized into a direct-type backlight and an edge-light type backlight according to the position of the light source. In the direct-type backlight, a large number of light sources such as cold cathode lamps or LEDs (light emitting diodes) are provided just under a liquid crystal panel, in which light emitted from the light sources is diffused through plural optical sheets and irradiated to the liquid crystal display device panel. In the edge-light type backlight, a light guide plate is used, in which light sources are provided at side edges, light incident from the light sources is diffused by the light guide plate and irradiated to the liquid crystal display panel through the plural optical sheets.
FIG. 11 shows an example of an edge-light type optical module unit applying LEDs.
As shown in FIG. 11, a plural number of LEDs 16 as light sources such as positional light sources are arranged in a side surface portion of a light guide plate 13. A diffusion sheet 12 is arranged above the light guide plate 13. The diffusion sheet 12 widely diffuses light emitted from the light guide plate 13.
Moreover, a first prism sheet 20 is arranged above the diffusion sheet 12, a second prism sheet 21 is arranged above the first prism sheet 20 and an optical sheet 22 is arranged above the second prism sheet 21, which respectively collect light to a visual direction to realize high luminance. Furthermore, a reflective sheet 17 is arranged below the light guide plate 13, which returns light leaking to a lower direction from the light guide plate 13 to the light guide plate 13 again for efficiently utilizing the light.
FIG. 12 shows a structure of a related-art light guide plate 13 described in Patent Literature 1. A brief explanation of FIG. 12 will be made. The light guide plate 13, light sources including the LEDs 16 which are light generating portions arranged at both end portions of the light guide plate 13 and a group of plural combined sheets 18 which guides light from the light sources to liquid crystal cells are mainly included. Plural prisms 14a to 14c are formed at given intervals on a reflection prism surface (lower surface) of the light guide plate 13.
Cross-sectional shapes of the plural prisms 14a to 14c formed on the reflection prism surface are determined in consideration of optical paths of direct light from the LEDs 16 and reflected light emitted from the LEDs 16 and reflected on a light emitting surface. That is, when the light from the LEDs 16 is emitted to the combined sheet group 18, two types of lights exist, which are a light 25 directly proceeding to the reflection prism surface (lower surface) from the LED 16 and alight 26 proceeding to the reflection prism surface after emitted from the LED 16 and reflected on the light emitting surface once. In order to guide the light to the combined sheet group 18 efficiently, it is necessary to determine the cross-sectional shapes of prisms in consideration of these two optical paths.
A cross-sectional shape of the prism 14a at the approximately the center is an isosceles triangle in which a point angle (T°) is 100 degrees and an inclination angle (a1) is 40 degrees. A depth of the prism 14a is D1.
A cross-sectional shape of the prism 14b positioned at ¼ of the light guide plate 13 from the end thereof is a triangle in which a point angle (T°) is 100 degrees and an inclination angle (a2) is 34.8 degrees. A depth of the prism 14b is D2.
A cross-sectional shape of the prism 14c positioned at the end of the light guide plate 13 is a triangle in which a point angle (T°) is 100 degrees and an inclination angle (a3) is 24.05 degrees. A depth of the prism 14c is D3.
That is, the shapes of the prisms are set so that the tip angles T of the prisms 14a to 14c are fixed in all prisms and the inclination angles of the prisms 14a to 14c are increased from a position closer to the LED 16 toward the center, and so that the depths of the prisms become deeper from the position closer to the LED 16 toward the center.
In the structure of Patent Literature 1, the plural prisms 14a to 14d formed on the reflection prism surface of the light guide plate 13 respectively have cross-sectional shapes determined in consideration of optical paths of the direct light from the LEDs 16 and the reflected light emitted from the LEDs 16 and reflected on the light emitting surface.
Accordingly, not only the direct light from the LEDs 16 but also the reflected light emitted from the LEDs 16 and reflected on the light emitting surface can be emitted approximately perpendicularly to the light emitting surface of the light guide plate 13.
As a result, it is possible to emit light from the light guide plate 13 efficiently. However, in order to secure viewing angle characteristics and suppressing luminance non-uniformity in an angular direction as well as to improve dimming characteristics (area light-emitting characteristics), it is difficult to satisfy conflicting characteristics of viewing angle characteristics, luminance non-uniformity and dimming characteristics as the tip angle T of the prisms is one kind, and it is necessary to use two pieces of expensive prism sheets, which causes problems that the number of components in the unit is increased and the assembly becomes complicated.
Here, dimming characteristics are area light-emitting characteristics, in which the backlight is divided into plural areas and each area light emission is independently controlled, thereby improving the contrast ratio so that black potions in a screen do not emit light and bright portions emit light.